Audio decoding apparatus and method for band expansion with aliasing suppression

ABSTRACT

A wideband, high quality audio signal is decoded with few calculations at a low bitrate. Unwanted spectrum components accompanying sinusoidal signal injection by a synthesis subband filter built with real-value operations are suppressed by inserting a suppression signal to subbands adjacent to the subband to which the sine wave is injected. This makes it possible to inject a desired sinusoid with few calculations.

TECHNICAL FIELD

The present invention relates to a decoding apparatus and decodingmethod for an audio bandwidth expansion system for generating a widebandaudio signal from a narrowband audio signal by adding additionalinformation containing little information, and relates to technologyenabling this system to provide high audio quality playback with fewcalculations.

BACKGROUND ART

Many audio encoding technologies for encoding an audio signal to a smalldata size and then reproducing the audio signal from the coded bitstreamare known. The international ISO/IEC 13818-7 (MPEG-2 AAC) standard inparticular is known as a superior method enabling high audio qualityplayback with a small code size. This AAC coding method is also used inthe more recent ISO/IEC 14496-3 (MPEG-4 Audio) system.

Audio coding methods such as AAC convert a discrete audio signal fromthe time domain to a signal in the frequency domain by sampling thetime-domain signal at specific time intervals, splitting the convertedfrequency information into plural frequency bands, and then encoding thesignal by quantizing each of the frequency bands based on an appropriatedata distribution. For decoding, the frequency information is recreatedfrom the code stream, and the playback sound is obtained by convertingthe frequency information to a time domain signal. If the amount ofinformation supplied for encoding is small (such as in low bitrateencoding), the data size allocated to each of the segmented frequencybands in the coding process decreases, and some frequency bands may as aresult contain no information. In this case the decoding processproduces playback audio with no sound in the frequency component of thefrequency band containing no information.

In general, because sensitivity to sound with a frequency aboveapproximately 10 kHz is lower than to sound at lower frequencies, highfrequency component data is generally dropped to provide narrowbandaudio playback if the audio coding scheme distributes information by aprocess based on human auditory perception.

If data is supplied at a bitrate of approximately 96 kbps, even the AACmethod can code a 44.1 kHz stereo signal to an approximately 16 kHzband, but if data is encoded with data supplied at half this rate, i.e.,48 kbps, the bandwidth that can be quantified and coded whilemaintaining sound quality is reduced to at most approximately 10 kHz. Inaddition to being narrowband, playback sound coded with a low 48 Kbpsbitrate also sounds cloudy.

A method enabling wideband playback by adding a small amount ofadditional information to a code stream for narrowband audio playback isdescribed, for example, in the Digital Radio Mondiale (DRM) SystemSpecification (ETSI TS 101 980) published by the EuropeanTelecommunication Standards Institute (ETSI). Similar technology knownas SBR (spectral band replication) is described, for example, in AES(Audio Engineering Society) convention papers 5553, 5559, 5560 (112thConvention, 2002 May 10–13, Munich, Germany).

FIG. 2 is a schematic block diagram of an example of a decoder for bandexpansion using SBR. Input bitstream 206 is separated by the bitstreamdemultiplexer 201 into low frequency component information 207, highfrequency component information 208, and sine wave-adding information209. The low frequency component information 207 is, for example,information encoded using the MPEG-4 AAC or other coding method, and isdecoded by the low-band decoder 202 whereby a time signal representingthe low frequency component is generated. This time signal representingthe low frequency component is separated into multiple (M) subbands byanalysis filter bank 203 and input to high frequency signal generator204.

The high frequency signal generator 204 compensates for the highfrequency component lost due to bandwidth limiting by copying the lowfrequency subband signal representing the low frequency component to ahigh frequency subband. The high frequency component information 208input to the high frequency signal generator 204 contains gaininformation for the compensated high frequency subband so that gain isadjusted for each generated high frequency subband.

An additional signal generator 211 generates injection signal 212whereby a gain-controlled sine wave is added to each high frequencysubband. The high frequency subband signal generated by the highfrequency signal generator 204 is then input with the low frequencysubband signal to the synthesis filter bank 205 for band synthesis, andoutput signal 210 is generated. The subband count on the synthesisfilter bank side does not need to be the same as the number of subbandson the analysis filter bank side. For example, if in FIG. 2 N=2M, thesampling frequency of the output signal will be twice the samplingfrequency of the time signal input to the analysis filter bank.

In this configuration the information contained in the high frequencycomponent information 208 or sine wave-adding information 209 relatesonly to gain control, and the amount of required information istherefore very small compared with the low frequency componentinformation 207, which also contains spectral information. This methodis therefore suited to encoding a wideband signal at a low bitrate.

The synthesis filter bank 205 in FIG. 2 is composed of filters that takeboth real number input and imaginary number input for each subband, andperform a-complex-valued calculation.

The decoder configured as above for band expansion has two filters, theanalysis filter bank and synthesis filter bank, performingcomplex-valued calculations, and decoding requires many calculations. Aproblem when the decoder is built for LSI devices, for example, is thatpower consumption increases and the playback time that is possible witha given power supply capacity decreases. Because the signals that wehear in the output from the synthesis filter bank are real-numbersignals, the synthesis filter bank may be configured with real numberfilter banks in order to reduce the calculations. While this reduces thenumber of calculations, if a sine wave is added using the same method aswhen the synthesis filter bank performs complex-valued calculations, apure sine wave is not actually added and the intended result is notachieved in the reproduced audio.

The present invention is therefore directed to solving these problems ofthe prior art, and provides a decoding apparatus and method for a bandexpansion system operating with few calculations by using a real-valuedcalculation filter bank whereby the intended audio playback is achievedby adding slight change to an added sine wave generation signal such aswould be inserted to a complex-valued calculation filter bank.

SUMMARY OF THE INVENTION

The present invention provides an audio decoding apparatus for decodingan audio signal from a bitstream,

-   -   the bitstream containing encoded information about a narrowband        audio signal and additional information for expanding the        narrowband signal to a wideband signal, and        -   the additional information containing high frequency            component information denoting a feature of a higher            frequency band than the band of the encoded information, and            sinusoid-adding information denoting a sinusoidal signal            added to a specific frequency band,

the audio decoding apparatus comprising:

a bitstream demultiplexer for demultiplexing the encoded information andadditional information from the bitstream;

a decoding means for decoding a narrowband audio signal from thedemultiplexed encoded information;

an analysis subband filter for separating the narrowband audio signalinto multiple first subband signals;

a high frequency signal generator for generating multiple second subbandsignals in a higher frequency band than the band of the encodedinformation from at least one first subband signal and high frequencycomponent information from the demultiplexed additional information;

a sinusoidal signal addition means for adding a sinusoidal signal to aspecific subband of the multiple second subband signals based on thesinusoid-adding information of the demultiplexed additional information;

a compensation signal generator for generating, based on the phasecharacteristic and amplitude characteristic of the sinusoidal signal, acompensation signal for suppressing aliasing component signals producedin subbands near a specific subband as a result of adding a sinusoidalsignal; and

a real-valued calculation synthesis subband filter for combining thefirst subband signals and second subband signals to obtain a widebandaudio signal.

Thus comprised, high quality audio playback can be achieved at a lowbitrate using few calculations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing an example of an audiodecoding apparatus according to the present invention;

FIG. 2 shows an example of the configuration of a prior art audiodecoding apparatus;

FIG. 3 shows an example of an additional signal generator for describingthe principle of the present invention;

FIG. 4 shows an example of an additional signal generator in a firstembodiment of the present invention;

FIGS. 5A and 5B, each shows an example of an injected complex-valuesignal;

FIG. 6 shows examples of the injection signals generated by theadditional signal generator shown in FIG. 3;

FIG. 7 shows only the real-number part of the injection signalsgenerated by the additional signal generator shown in FIG. 3;

FIG. 8 shows examples of injection signals and compensation signalsgenerated by the additional signal generator and compensation signalgenerator shown in FIG. 4;

FIG. 9 is a spectrum diagram for when a sine wave for only thereal-value part is injected to the real-value synthesis filter;

FIG. 10 is a spectrum diagram for when a sine wave for only thereal-value part and a compensation signal are injected to the real-valuesynthesis filter;

FIG. 11 shows another example of the injection signal and compensationsignal shown by way of example in FIG. 8;

FIG. 12 shows an example of the additional signal generator in a secondembodiment of the present invention; and

FIG. 13 is a block diagram showing the principle of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 13 is a block diagram showing the principle of the presentinvention. Music and other audio signals contain a low frequency bandcomponent and a high frequency band component. Encoded audio signalinformation is carried by the low frequency band component, and toneinformation (sinusoidal information) and gain information are carried bythe high frequency band component. The receiver decodes the audio signalfrom the low frequency band component, but for the high frequency bandcomponent, copies and processes the low frequency band component usingthe tone information and gain information to synthesize a pseudo-audiosignal. Phase information and amplitude information are needed tosynthesize this pseudo-audio signal, and synthesis thus requires acomplex-valued calculation. Because complex-valued calculations requireoperations on both the real number and imaginary number parts, thecalculation process is complex and time-consuming. To simplify thiscalculation process the present invention operates using only the realnumber part. However, if the calculations are done using only thereal-value part for certain subbands, noise signals appear in theadjacent higher and lower subbands. A compensation signal for cancellingthese noise signals is generated using the phase information, amplitudeinformation, and timing information contained in the tone information.

An audio decoding apparatus and method according to a preferredembodiment of the present invention are described below with referenceto the accompanying figures.

(Embodiment 1)

FIG. 1 is a schematic diagram showing a decoding apparatus performingbandwidth expansion by means of spectral band replication (SBR) based ona first embodiment of the present invention.

The input bitstream 106 is demultiplexed by the bitstream demultiplexer101 into low frequency component information 107, high frequencycomponent information 108, and sine signal-adding information 109. Thelow frequency component information 107 is information that is encodedusing, for example, the MPEG-4 AAC coding method, is decoded by the lowfrequency decoder 102, and a time signal representing the low frequencycomponent is generated. The resulting time signal representing the lowfrequency component is then divided into multiple (M) subbands by theanalysis filter bank 103, and input to the bandwidth expansion means(high frequency signal generator) 104. The high frequency signalgenerator 104 copies the low frequency subband signal representing thelow frequency component to a high frequency subband to compensate forthe high frequency component lost by the bandwidth limit. The highfrequency component information 108 input to the high frequency signalgenerator 104 contains gain information for the high frequency subbandto be generated, and the gain is adjusted for each generated highfrequency subband.

Additional signal generator 111 produces injection signal 112 so that again-controlled sine wave is added to each high frequency subbandaccording to the sine signal-adding information (also called toneinformation) 109. The high frequency subband signals generated by thehigh frequency signal generator 104 are input with the low frequencysubband signals to the synthesis filter bank 105 for band synthesis,resulting in output signal 110. The number of subbands on the synthesisfilter bank does not need to match the number of subbands on theanalysis filter bank side. For example, if in FIG. 1 N=2M, the samplingfrequency of the output signal will be twice the sampling frequency ofthe time signal input to the analysis filter bank.

The input bitstream 106 contains narrowband encoded information for theaudio signal (i.e., low frequency component information 107) andadditional information for expanding this narrowband signal to awideband signal (i.e., high frequency component information 108 and sinesignal-adding information 109).

The synthesis filter bank 105 of the decoding apparatus shown in FIG. 1is composed of real-valued calculation filters. It will also be obviousthat a complex-valued calculation filter that can perform real-valuedcalculations could be used.

The decoding apparatus shown in FIG. 1 also has a compensation signalgenerator 114 for generating compensation signal 113 for compensatingthe difference resulting from sinusoidal signal addition.

The input bitstream 106 is demultiplexed by the bitstream demultiplexer101 into low frequency component information 107, high frequencycomponent information 108, and sine signal-adding information 109.

The low frequency component information 107 is, for example, an MPEG-4AAC, MPEG-1 Audio, or MPEG-2 Audio encoded bitstream that is decoded bya low frequency decoder 102 having a compatible decoding function, and atime signal representing the low frequency component is generated. Theresulting time signal representing the low frequency component is thendivided into multiple (M) first subbands S1 by the analysis filter bank103, and input to the high frequency signal generator 104. The analysisfilter bank 103 and synthesis filter bank 105 described below are builtfrom a polyphase filter bank or MDCT converter. Band splitting filterbanks are known to one with ordinary skill in the related art.

The first subband signals S1 for the low frequency signal component fromthe analysis filter bank 103 are output directly by the high frequencysignal generator 104 and also sent to the synthesis part. The highfrequency signal generation part of the high frequency signal generator104 receives the first subband signals S1 and using high frequencycomponent information 108, injection signal 112, and compensation signal113 generates multiple second subband signals S2. The second subbandsignals S2 are in a higher frequency band than the first subband signalsS1. The high frequency component information 108 includes informationindicating which one of the first subband signals S1 is to be copied,and which one of the second subband signals S2 is to be generated, andgain control information indicating how much the copied first subbandsignal S1 should be amplified.

If there is no sine signal-adding information 109 or no signal actuallygenerated using the sine signal-adding information 109, the synthesisfilter bank 105 with N (where N is greater or equal to M) subbandsynthesis filters combines the expanded-bandwidth subband signals outputfrom the high frequency signal generator 104 and the low frequencysignal component from the analysis filter bank 103 to produce widebandoutput signal 110.

In this first embodiment of the invention the synthesis filter bank 105is a real-value calculation filter bank. That is, the synthesis filterbank 105 does not use imaginary number input, only has a real numberinput part, and uses filters that perform real-valued calculations. Thissynthesis filter bank 105 is therefore simpler and operates faster thana filter that operates with complex-valued calculations.

If there is sine signal-adding information 109, the sine signal-addinginformation 109 is input to the additional signal generator 111 wherebyinjection signal 112 is generated, and added to the output signal fromhigh frequency signal generator 104. The sine signal-adding information109 is also input to the compensation signal generator 114 wherebycompensation signal 113 is produced, and similarly added to the outputsignal of high frequency signal generator 104.

The output signal from high frequency signal generator 104 is input tosynthesis filter bank 105. The synthesis filter bank 105 outputs outputsignal 110 regardless of whether there is an added signal based on sinesignal-adding information 109.

Generating the injection signal 112 and compensation signal 113 based onsine signal-adding information 109 is described in further detail belowusing FIG. 3 and FIG. 4.

FIG. 3 shows the additional signal generator 111 used in the audiodecoding method describing the basic principle of the present invention,and FIG. 4 shows the additional signal generator 111 and compensationsignal generator 114 in a first embodiment of the present invention.

The additional signal generator 111 is described first with reference toFIG. 3. The information contained in the sine signal-adding information109 includes injected subband number information denoting to whichsynthesis filter bank the sine wave is injected, phase informationdenoting the phase at which the injected sinusoidal signal starts,timing information denoting the time at which the injected sinusoidalsignal starts, and amplitude information denoting the amplitude of theinjected sinusoidal signal.

Injected subband information extraction means 406 extracts the injectedsubband number. The phase information extraction means 402 determines,based on the phase information if phase information is contained in thesine signal-adding information 109, the phase at which the injectedsinusoidal signal starts. If phase information is not contained in thesine signal-adding information 109, the phase information extractionmeans 402 determines the phase at which the injected sinusoidal signalstarts with consideration for continuity to the phase of the previoustime frame.

Amplitude extraction means 403 extracts the amplitude information.Timing extraction means 404 extracts the timing information indicatingwhat time to start sine wave injection and what time to end injectionwhen a sine wave is injected to the synthesis filter bank.

Based on the information from the phase information extraction means402, amplitude extraction means 403, and timing extraction means 404,the sinusoid generating means 405 generates the sine wave (tone signal)to be injected. It should be noted that the frequency of the generatedsine wave can be desirably set to, for example, the center frequency ofthe subband or a frequency offset a predetermined offset from the centerfrequency. Further, the frequency could be preset according to thesubband number of the injected subband. For example, a sine wave of theupper or lower frequency limit of the subband could be generatedaccording to whether the subband number is odd or even. It is assumedbelow that a sine wave with the center frequency of the subband isproduced, i.e., a periodic signal with four subband signal samplingperiods is produced.

The sine wave injection means 407 inserts the sine wave output bysinusoid generating means 405 to the synthesis filter subband matchingthe number acquired by the injected subband information extraction means406. The output signal from sine wave injection means 407 is injectionsignal 112.

Consider a complex-valued signal with four periods and amplitude Sinjected to subband K as shown in the table in FIG. 6. The valuesdenoted (a,b) in the table mean the complex-valued signal a+jb. where jis an imaginary value. Referring to FIG. 5A, the signal inserted tosubband K in FIG. 6 is a periodic signal that changes 501, 502, 503, 504in FIG. 5A due to the relationship between the real-value part and theimaginary value part.

If, unlike in the present invention, the synthesis filter bank is afilter that takes complex-valued input and performs complex-valuedcalculations, the output signal of the decoding system obtained by thisinjection signal has a single frequency spectrum and a so-called puresine wave is injected. However, if the synthesis filter bank is a filterthat takes only real-value input and performs only real-valuecalculations as in the present invention, a real-number signal notcontaining the imaginary number part shown in FIG. 6 is injected tosubband K as shown in FIG. 7. With this injection signal the decodingsystem using a synthesis filter that takes only real values outputs asingle frequency spectrum as shown in FIG. 9 (spectrum 902 of theinjected sine wave) and unwanted spectrums in the bands above and belowthe sine wave spectrum (unwanted spectrum 903). This is because asynthesis filter using real-valued calculation cannot completelyeliminate spectrum leakage into adjacent subbands due to the filtercharacteristics, and these spectrum leaks appear as aliasing components.

By providing a compensation signal generator 114 as shown in FIG. 4 inaddition to the additional signal generator 111 shown in FIG. 3 in asynthesis filter bank using real-valued calculation with only real valueinput, the unwanted spectrum components shown in FIG. 9 can be removed.

Additional signal generator 111 and compensation signal generator 114according to the present invention are described next with reference toFIG. 4. In FIG. 4 the sine signal-adding information 109, phaseinformation extraction means 402, amplitude extraction means 403, timingextraction means 404, sinusoid generating means 405, injected subbandinformation extraction means 406, sine wave injection means 407, andinjection signal 408 are the same as described with reference to FIG. 3.What differs from FIG. 3 is the addition of compensation subbandinformation determining means 409 and compensation signal generator 410.

The compensation subband information determining means 409 determinesthe subband to be compensated based on the information obtained by theinjected subband information extraction means 406 indicating the numberof the synthesis filter bank to which the sine wave is injected. Thesubband to be compensated is a subband near the subband to which thesine wave is injected, and may be a high frequency subband or lowfrequency subband. The high frequency subband and low frequency subbandto be compensated will vary according to the characteristics of thesynthesis filter bank 105, but are here assumed to be the subbandsadjacent to the subband of the injected sine wave. For example, when thesine wave is injected to subband K, subband K+1 and subband K−1 are,respectively, the high frequency subband and low frequency subband to becompensated.

The compensation signal generator 410 generates a signal cancellingaliasing spectra in the compensated subband based on the output of phaseinformation extraction means 402, amplitude extraction means 403, andtiming extraction means 404, and outputs this signal as compensationsignal 113. This compensation signal 113 is added to the input signal tothe synthesis filter bank 105 in the same way as injection signal 112.The amplitude S and phase of the compensation signal 113 are adjustedfor subband K−1 and subband K+1 as shown in the table in FIG. B.

In FIG. 8 Alpha and Beta are values determined according to thecharacteristics of the specific synthesis filter bank, and morespecifically are determined with consideration for the amount ofspectrum leakage to adjacent subbands in the filter bank.

As will be known from FIG. 8, if a sinusoidal signal is added to subbandK, the amplitude of a sinusoidal signal of cycle period T is amplitude Sat time 0, amplitude 0 at time 1T/4, amplitude −S at time 2T/4, andamplitude 0 at time 3T/4. A compensation signal is applied to subbandK−1 and subband K+1. In the drawings, TIMEs 0, 1, 2 and 3 correspond totimes 0, 1T/4, 2T/4 and 3T/4, respectively.

The compensation signal applied to subband K−1 has amplitude 0 at time0, amplitude Alpha*S at time 1T/4, amplitude 0 at time 2T/4, andamplitude Beta*S at time 3T/4.

The compensation signal applied to subband K+1 has amplitude 0 at time0, amplitude Beta*S at time 1T/4, amplitude 0 at time 2T/4, andamplitude Alpha*S at time 3T/4.

FIG. 10 is a spectrum graph for the sine wave injected by a preferredembodiment of this invention. As will be known from FIG. 10, theunwanted spectrum component 903 observed in FIG. 9 is suppressed.

By introducing this compensation signal, unwanted spectrum componentsare not produced even if a sinusoidal signal is injected to a real-valuefilter bank, and a sine wave can be injected to a desired subband withminimal calculations.

The invention has been described with reference to a sinusoidal signalinjected to subband K where the initial phase is 0 and either thereal-value part or imaginary-value part goes to 0 as shown in FIG. 5A.As shown in FIG. 5B, however, the present invention can also be appliedwhen the phase is shifted δ from the state shown in FIG. 5A. Therelationship between the injection signal and compensation signal inthis case can be expressed as shown in the table in FIG. 11, forexample, where S, P, and Q are values determined according to thecharacteristics of the filter bank with consideration for the amount ofspectrum leakage by the filter bank to adjacent subbands.

Furthermore, for a subband K to which the sine wave is injected acompensation signal is injected to adjacent subbands K−1 and K+1, butadjacent subbands other than K−1 and K+1 may need correction dependingon the characteristics of the synthesis filter. In this case thecompensation signal is simply injected to the subbands that needcorrection.

(Embodiment 2)

FIG. 12 is a schematic diagram showing an additional signal generator ina second embodiment of the present invention. This additional signalgenerator differs from the additional signal generator, 111 shown inFIG. 4 in that interpolated information 1201 calculated by the sinusoidgenerating means 405 is input to compensation signal generator 410 sothat the compensation signal 113 is calculated based on the interpolatedinformation 1201.

The sinusoid generating means 405 in the above first embodiment adjuststhe amplitude of the generated sine wave based only on the amplitudeinformation of the current frame extracted by the amplitude extractionmeans 403. The sinusoid generating means 405 of this second embodiment,however, interpolates the amplitude information using amplitudeinformation from neighboring frames, and adjusts the amplitude of thegenerated sine wave based on this interpolated amplitude information.

Because the amplitude of the generated sine wave changes smoothly as aresult of this process, the observed sound quality of the output signalcan be improved.

Because the amplitude of the generated sine wave is changed byinterpolation with this configuration, the amplitude of thecorresponding compensation signal must also be adjusted. Therefore, theinterpolated information output by the sinusoid generating means 405 isalso input to the compensation signal generator 410 to adjust theamplitude of the compensation signal 113 synchronized to theinterpolated variable amplitude of the sine wave.

This configuration of the invention can correctly calculate thecompensation signal and suppress unwanted spectrum components even whenthe amplitude of the generated sine wave is interpolated.

It will also be apparent that the process of the audio decodingapparatus shown in FIG. 1 can also be written in software using aprogramming language. In addition, this software program can be recordedto and distributed by a data recording medium.

When using a synthesis filter bank that reduces the number of operationsby using only real-valued calculations, unwanted spectrum componentsaccompanying sine wave addition can be suppressed and only the desiredsine wave can be injected by injecting a compensation signal to the lowfrequency or high frequency subband of the subband to which the sinewave is added.

1. An audio decoding apparatus for decoding an audio signal from abitstream containing encoded information about a narrowband audio signaland additional information for expanding the narrowband audio signal toa wideband audio signal, the additional information containing highfrequency component information denoting a feature of a frequency bandhigher than a frequency band of the encoded information narrowband audiosignal, and sinusoid-adding information denoting a sinusoidal signaladded to a specific frequency band, said audio decoding apparatuscomprising: a bitstream demultiplexer operable to demultiplex theencoded information and the additional information from the bitstream; adecoder operable to decode the narrowband audio signal from thedemultiplexed encoded information; an analysis subband filter operableto separate the decoded narrowband audio signal into a first subbandsignal composed of a plurality of subband signals; a sinusoidal signalgenerator operable to generate a sinusoidal signal added to a specificsubband at a frequency band higher than a frequency band of the encodedinformation of the narrowband audio signal based on the sinusoid-addinginformation in the demultiplexed additional information; a correctionsignal generator operable to generate, based on a phase characteristicand an amplitude characteristic of the sinusoidal signal, a correctionsignal added to subbands near a specific subband to suppress aliasingcomponent signals occurring in the subbands near the specific subband; ahigh frequency signal generator operable to generate a second subbandsignal composed of a plurality of subband signals in a frequency bandhigher than the frequency band of the encoded information of thenarrowband audio signal from the first subband signal and high frequencycomponent information in the demultiplexed additional information, andoperable to add the sinusoidal signal and correction signal to thesecond subband signal; and a real-valued calculation subband synthesisfilter operable to combine the first subband signal and the secondsubband signal to obtain the wideband audio signal.
 2. An audio decodingapparatus according to claim 1, wherein the aliasing component signalscontain at least components suppressed after synthesis by a subbandsynthesis filter that performs complex-valued calculations.
 3. An audiodecoding apparatus according to claim 1, wherein the first subbandsignal is composed of low frequency subband signals, and the secondsubband signal is composed of high frequency subband signals.
 4. Anaudio decoding apparatus according to claim 1, wherein the correctionsignal generated by the correction signal generator suppresses aliasingcomponent signals produced in a subband adjacent to the subband to whichthe sinusoidal signal is added.
 5. An audio decoding apparatus accordingto claim 4, wherein when the sinusoidal signal is added to subband K, asinusoidal signal of period T has amplitude S at time 0 amplitude 0 attime 1T/4, amplitude −S at time 2T/4, and amplitude 0 at time 3T/4, andcorrection signals are applied to subband K−1 and subband K+1; thecorrection signal applied to subband K−1 has amplitude 0 at time 0,amplitude Alpha*S at time 1T/4, amplitude 0 at time 2T/4, and amplitudeBeta*S at time 3T/4; and the correction signal applied to subband K+1has amplitude 0 at time 0, amplitude Beta*S at time 1T/4, amplitude 0 attime 2T/4, and amplitude Alpha*S at time 3T/4; where Alpha and Beta areconstants.
 6. An audio decoding apparatus according to claim 1, whereinan amplitude of the correction signal generated by the correction signalgenerator is synchronously adjusted to the amplitude characteristic ofthe sinusoidal signal.
 7. An audio decoding method for decoding an audiosignal from a bitstream containing encoded information about anarrowband audio signal and additional information for expanding thenarrowband audio signal to a wideband audio signal, and the additionalinformation containing high frequency component information denoting afeature of a frequency band higher than a frequency band of the encodedinformation of the narrowband audio signal, and sinusoid-addinginformation denoting a sinusoidal signal added to a specific frequencyband, said audio decoding method comprising: demultiplexing the encodedinformation and the additional information from the bitstream; decodingthe narrowband audio signal from the demultiplexed encoded information;separating the decoded narrowband audio signal into a first subbandsignal composed of a plurality of subband signals; generating asinusoidal signal added to a specific subband at a frequency band higherthan a frequency band of the encoded information of the narrowband audiosignal based on the sinusoid-adding information in the demultiplexedadditional information; generating, based on a phase characteristic andan amplitude characteristic of the sinusoidal signal, a correctionsignal added to subbands near a specific subband to suppress aliasingcomponent signals occurring in the subbands near the specific subband;generating a second subband signal composed of a plurality of subbandsignals in a frequency band higher than the frequency band of theencoded information of the narrowband audio signal from the firstsubband signal and high frequency component information in thedemultiplexed additional information, and adding the sinusoidal signaland correction signal to the second subband signal; and synthesizing thefirst subband signal and the second subband signal using a real-valuedcalculation to obtain the wideband audio signal.
 8. An audio decodingmethod according to claim 7, wherein the aliasing component signalscontain at least components suppressed after synthesis performed usingcomplex-valued calculations.
 9. An audio decoding method according toclaim 7, wherein the first subband signal is composed of low frequencysubband signals, and the second subband signal is composed of highfrequency subband signals.
 10. An audio decoding method according toclaim 7, wherein the generated correction signal suppresses aliasingcomponent signals produced in a subband adjacent to the subband to whichthe sinusoidal signal is added.
 11. An audio decoding method accordingto claim 10, wherein when the sinusoidal signal is added to subband K, asinusoidal signal of period T has amplitude S at time 0, amplitude 0 attime 1T/4, amplitude −S at time 2T/4, and amplitude 0 at time 3T/4, andcorrection signals are applied to subband K−1 and subband K+1; thecorrection signal applied to subband K−1 has amplitude 0 at time 0,amplitude Alpha*S at time 1T/4, amplitude 0 at time 2T/4, and amplitudeBeta*S at time 3T/4; and the correction signal applied to subband K+1has amplitude 0 at time 0, amplitude Beta*S at time 1T/4, amplitude 0 attime 2T/4, and amplitude Alpha*S at time 3T/4; where Alpha and Beta areconstants.
 12. An audio decoding method according to claim 7, wherein anamplitude of the generated correction signal is synchronously adjustedto the amplitude characteristic of the sinusoidal signal.
 13. Acomputer-readable medium having stored thereon a program comprisingcomputer executable code operable to cause a computer to perform theaudio decoding method claimed in claim 7.